Heretofore, connecting rods interconnecting piston pins and crank pins have widely been used in vehicular engines. Each of the connecting rods has a larger end coupled to the crankpin and a smaller end coupled to the piston pin. For manufacturing a connecting rod, it is generally customary to produce a one-piece connecting rod having a larger end and a smaller end by forging or the like, and thereafter to fracture the larger end into a cap and a rod.
The cap and the rod are rejoined by bolts into a connecting rod. After the connecting rod is inspected, it is assembled as an engine component in a vehicle engine. When the cap and the rod are rejoined together, if the joint area between the cap and the rod contains a step exceeding a predetermined tolerance, then the connecting rod is rejected as a nonconforming product.
Generally, there are known apparatus for inspecting a step surface with a laser beam. For example, Japanese Laid-Open Patent Publication No. 11-156577 discloses a butting position detecting apparatus for applying a laser beam obliquely from a slit laser oscillator to a step between workpieces WA, WB, detecting a reflected beam with a CCD line sensor, and detecting a butting position at the step between the workpieces WA, WB based on a change from a higher level to a lower level of the amount of light detected by the CCD line sensor.
Japanese Laid-Open Patent Publication No. 2001-183113 reveals a step detecting apparatus having an illuminating device for irradiating a certain area of an inspected object such as a letter or the like with a laser beam, a dichroic mirror, a CCD sensor, etc. The step detecting apparatus reads a step such as of a surface irregularity of the inspected object and performs predetermined image processing on the read data.
Japanese Laid-Open Patent Publication No. 2003-315020 discloses a portable beam-interruption measuring apparatus having a guide for contacting the surface of a measured object so as to serve as a support, the guide being positioned eccentrically with respect to a beam slit. The disclosed portable beam-interruption measuring apparatus is capable of avoiding an error which would be caused if the angle of the apparatus with respect to the measured object changes, and of measuring a gap and a step in the measured object while the slit beam is applied to the measured object at a constant angle and an image capturing device captures an image at a constant angle.
The apparatus disclosed in Japanese Laid-Open Patent Publications Nos. 11-156577, 2001-183113, and 2003-315020 are based on the assumption that the measured surfaces are flat and the step to be measured is provided by the flat surfaces. However, when the larger end of a connecting rod is fractured into a cap and a rod and then the cap and the rod are rejoined, a step provided in the joint area between the cap and the rod has a curved surface. The apparatus disclosed in the above publications is not suitable for use in inspecting the step in such a joint area between the cap and the rod because the apparatus tend to produce measurement errors.
Specifically, the apparatus disclosed in the above publications are designed to measure the extent of a step provided by flat or slanted surfaces. On the other hand, when the larger end of a connecting rod is fractured into a cap and a rod, the cap and the rod will shrink due to residual stresses produced after the fracture. Since the cap and the rod shrink, the joint area subsequently formed between the cracked surface of the cap and the cracked surface of the rod when the cap and the rod are rejoined includes a step having a curved surface. Various cracked connecting rods include steps having various differently shaped curved surfaces. The surface roughness of the measured surfaces needs to be taken into account. Another problem is that when the larger end of a connecting rod is fractured into a cap and a rod, the cap and the rod may possibly expand due to residual stresses produced after the fracture.
Consequently, if the apparatus disclosed in the above publications are applied to inspect the step in the joint area between the cap and the rod, then the step cannot be measured or the measured data contains a large error because the measured surface is not constant, but includes irregularities.
It is a general object of the present invention to provide an apparatus for accurately inspecting the extent of a step in the joint area of a connecting rod which is formed when a rod and a cap that are produced by fracturing the larger end of the connecting end are rejoined.
It is a main object of the present invention to provide a method of accurately inspecting the extent of a step in the joint area of a connecting rod which is formed when a rod and a cap that are produced by fracturing the larger end of the connecting end are rejoined.